In the semiconductor industry, fuse elements are widely used features in integrated circuits for a variety of purposes, such as improving manufacturing yield or customizing a generic integrated circuit. For example, by replacing defective circuits on a chip with redundant circuits on the same chip, manufacturing yields can be significantly increased. Replacing defective circuits is especially useful for improving manufacturing yield of the memory chips since memory chips consist of a lot of identical memory cells and cell groups. By selectively blowing fuses within an integrated circuit that has multiple potential uses, a generic integrated circuit design may be economically manufactured and adapted to a variety of custom uses.
There are two different ways to disconnect fuses. In one way, the disconnection is carried out by the action of a laser beam, and the fuse is referred to as a laser fuse. In another way, the disconnection is carried out by electrical destruction resulting from the production of heat. The fuse is referred as an electrical fuse, or E-fuse.
Laser programmable redundancy has been widely used in large-scale memory devices. However, the laser repair rate in various structures such as in lower level metal layers is low and the process is complex. FIG. 1 illustrates a laser fuse formed close to the surface of a chip. Device 6 is a laser fuse. Oxide 5 covers the fuse 6. If the fuse 6 is to be burned out from the top of the oxide 5 by a laser, the thickness T of the oxide 5 has to be within a certain range, for example, between about 0.1 kÅ to about 4.0 kÅ. Therefore, an extra mask is needed to form opening 4, and the process has to be precisely controlled. If a laser fuse 10 is in a lower level layer deep in a chip, as shown in FIG. 2, opening 8 will be deeper, while the thickness T of the oxide still has to be controlled precisely, which increases the complexity significantly and decreases the repairable rate.
In addition, as technology is scaling down to 0.13 μm or below, copper is implemented as interconnects or power lines. Copper is a material with high current density tolerance and is not easily burned out by using a laser gun. Furthermore, the combination of copper plus low-k material 12 (used as inter-layer dielectrics) is becoming a trend to improve RC delay. However, low-k material 12 cracks easily when etching the opening 8 in FIG. 2. This decreases the device reliability and increases the production cost.
Electrical fuses were developed to improve repairable rates. FIG. 3 illustrates a conventional electrical fuse 13. A polysilicon strip 15 is formed and patterned. The regions 14 and 16 of the polysilicon strip 15 are doped with p+ and n+ dopant. The central region 18 is left un-doped. A silicide 20 is formed over the polysilicon strip 15. Before the fuse 13 is burned out, its resistance is mainly determined by the resistance of the silicide 20 so that the resistance is low. When a predetermined programming potential is applied across the silicide layer 20 from nodes 22 and 24, the silicide layer 20 agglomerates to form an electrical discontinuity. Therefore the resistance of the fuse 13 is mainly determined by the underlying polysilicon strip 15 so that the resistance is significantly increased. The central un-doped region 18 makes the fuse resistance higher. The electrical fuse shown in FIG. 3 typically has a higher repairable rate than a laser fuse. However, the repairable rate is still not satisfactory. Additionally, the fuse of FIG. 3 is formed laterally and occupies more layout space.
There are several disadvantages faced by conventional methods of making fuses. Firstly, the repairable rate is typically low. Secondly, the additional masking layer needed for laser repair incurs higher costs. The process is also more complex with higher uncertainty. Thirdly, the structure design is not flexible. Fuses typically have to be designed in higher layers, as it is harder to form deep laser trenches through to lower layers. Therefore, new methods of designing e-fuses are needed.